Typically, a high power x-ray tube includes an evacuated envelope made of metal, ceramic or glass, which holds a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e., thermionic emission occurs. A high potential, typically on the order of 10-300 kilovolts, is applied between the cathode and an anode assembly, which also is located within the evacuated envelope. This potential causes electrons to flow from the cathode to the anode assembly through the evacuated region within the interior of the evacuated envelope. The electron beam strikes the anode with sufficient energy that x-rays are generated.
FIG. 1 and FIG. 2 illustrate an exemplary conventional x-ray tube 10 that includes a cathode 12 and an anode 14. Often, the anode material is tungsten with a thickness greater than 1 millimeter. Electrons 16 from the cathode 12 are accelerated and focused to a focal spot 18 on the incident (or front) surface of the anode 14, and x-rays 20 are emitted in all directions from this focal spot. A fraction of these x-rays emerge from the front anode surface and pass through a window 22 and collimator 24 aperture to constitute the emergent x-ray beam 26. Because both the incident electron beam and the emergent x-ray beam respectively enter and leave the same front anode surface, this anode is herein referred to as a reflection anode. Conventional x-ray tubes use reflection anodes because the x-rays emitted in the direction of the electron beam are mostly absorbed by the anode material, and only a small fraction is transmitted through the anode.
The geometric configuration of the cathode 12, anode 14 and collimator 24 for the x-ray tube 10 may be described by both the anode inclination (AI) angle and the x-ray emission (XE) angle, as shown in FIG. 1. The AI angle is defined as the angle between the axis of the incident electron beam and the normal, N, to the front anode surface. The XE angle is defined as the angle between the axes of the incident electron beam (this axis being projected or extended through the anode) and the emergent x-ray beam. In present day x-ray tubes, the AI angle typically is in the range of about 10 to 30 degrees, and the XE angle typically is equal to about 90 degrees.
The shape of the collimator aperture determines the shape of the x-ray beam. For example, a circular aperture provides a cone-shaped beam with its vertex at the focal spot and with the vertex half-angle equal to arc tan (r/d), where r is the radius of the collimator aperture and d is the distance of the plane of the collimator aperture from the focal spot.
In FIG. 1, a filter 28 is used at the collimator aperture for x-ray imaging applications in diagnostic radiology. This filter attenuates the x-rays primarily in the low energy region of the x-ray spectrum, and thereby reduces dose and noise to improve the quality of the x-ray image. The material and thickness of the filter depends on the x-ray attenuation properties of the object to be inspected. As a basis of comparing x-ray power outputs from different x-ray tubes, a standard filtration of 3 millimeters aluminum often is used to provide a conventional minimum filtration for diagnostic radiological procedures in medicine.
With the above features, many present day x-ray tubes have one or more of the following limitations: for cone shaped x-ray beams, the x-ray intensity distribution is asymmetric with respect to the beam axis; for cone shaped x-ray beams, the projected size of the focal spot becomes larger when observed at different points in the beam area (see focal spot 32 versus focal spot 34 in FIG. 2), which results in a “blooming effect” of the focal spot size; for cone shaped x-ray beams, the vertex half angle cannot exceed the anode inclination angle which ranges from 6 degrees to 30 degrees for present day high power x-ray tubes; and for high power tubes, the rotating anode has a large mass for high heat capacity, which imposes practical restrictions on the maximum anode diameter and rotation speed, and consequently on the maximum permissible input power and on the cooling rate that can be achieved.